Vo Lam

Abstract

Three varieties of sweet potato, Trang giay, Duyen ngoc and Hshinchu, were
grown for forage production with harvests at 15 to 20 day
intervals during a period of 125 days. Two kinds of fertiliser were used,
chemical fertiliser (160 kg N, 60 kg P and 30 kg K per ha), and goat manure
with a similar amount of N. The control treatment was without fertiliser.

Dry matter (DM) yield of sweet potato vines was similar for both sources of
fertilizer and was some 30% better than in the control with no fertilizer. The
highest DM content was in the control treatment with no fertiliser applied and
the lowest in the treatment with goat manure. The sweet potato vines fertilised with chemical fertiliser had the highest
CP content. Ash content of sweet potato vines fertilised with goat
manure was higher than on the two other treatments.
Sweet potato for forage production should be replanted after four months
and/or be given fertilizer to prolong the harvesting period.

Key words: Duyen ngoc, goat manure, Hshinchu, Trang giay

Introduction

Sweet potatoes are grown throughout the tropics for their edible tubers,
which are ·an important source of food in many developing countries for both
humans and livestock. In the Mekong River Delta region, sweet potato is well
adapted to all the agro-ecological zones and has been grown for
a long time in the area (Kim 2000). Most of the sweet potato is grown in the
rice based cropping systems, which are generally practised in areas
characterised by fertile and sandy soils and can thus produce a considerable
amount of tubers (Binh and Toca 1997).

The aerial part of sweet potato, the vines, may be utilised as an animal
feed in smallholder animal production systems (Phuc 2000). The productive
potential of different varieties of sweet potato can vary from 4.3 to 6.0
tonnes dry matter (DM) of vines per ha and crop (Dominguez 1992). When sweet
potato was planted as a perennial crop and the vines were cut at intervals
of 15 to 20 days large amounts of green feed for animals were produced (Son
1992). Recent research on the expanded use of sweet potato vines as a
forage/feed for ruminants has shown that there is a good potential, both for
agro-biological and economical reasons. The most important constraint for
using sweet potato vines is that the largest amount of sweet potato vines is
available during a short period of time from seasonal cropping of the
tubers. Ruiz et al (1981) showed that some varieties of sweet potato can be
grown two or three times per year and yield up to 125 tons of fresh biomass
per ha, out of which 64% constitutes the aerial part, and can be used as
forage. Binh (1997) reported that almost all farmers used chemical
fertiliser for their sweet potato in the Mekong River region and fertiliser
rates per ha were relatively high (250 kg urea, 250 kg P, and 50 kg K).
Animal manure was not commonly used in the area (Binh and Tacao 1997).
However, the use of goat manure for tuber crops has been reported to
increase DM production and improve soil fertility and water holding
capacity. Goat manure can substitute part of the N, P, and K fertiliser
(Duarsa et al 1996). However, little information is available on the effect
of goat manure on forage yield from sweet potato.
The objective of the experiment was to determine the aerial biomass
production of different varieties of sweet potato and the effect of goat
manure in comparison to chemical fertiliser on forage yield in the Mekong
Delta of Vietnam.

Materials and Methods

Location

The experiment was conducted from May to October, 2002 at the Research farm
of Can Tho University, Can Tho city, Vietnam.

Experimental design

The experiment was a 3 x 3 factorial experiment with four replicates (four
blocks) in a Randomised Complete Block Design.
The treatments were:

Variety:
Trang giay
Duyen ngoc
Hshinchu

Fertilizer:
Chemical NPK
Goat manure
None

The rate of fertiliser application was 60 kg N, 60 kg P and 30 kg K per ha
for chemical fertiliser and 60 kg N for goat manure. The control treatment was
without fertiliser.

The selected land was demarcated by thirty-six equal subplots, 5.6m wide
and 2.6m long in size and nine subplots were grouped into a block.

Plant material, planting and management

Of the three varieties of sweet potato used in the experiment, two are
typical for tuber production: Trang giay (trifoliate, 3 points/leaf) and
Duyen Ngoc (palmate, 5 points/leaf). The third variety (Hshinchu) had entire, 1
point/leaf and big area leaf. Sweet potato stems were brought from farmers'
fields and home gardens in the area and the stem cuttings were used for
planting. The selected stems were of similar size in diameter and number of
exposed nodes.

Two raised beds at the same site on arable land in the experimental farm
were selected to conduct the experiment. The size of each bed was 6 m wide
and 30 m long with a relatively uniform vegetation cover and even surface.
To determine the homogeneity of the soil, the soil was sampled before the
start of the experiment and at the last harvest. For the sampling before the
start six sites within the two selected beds were randomly sampled. For the
sampling at the end of the experiment, 10 points at 0- 30 cm of depth within
a subplot were taken and pooled to a general sample.

Land for the trial was cleared and the soil prepared to clods of 3-5 cm in
diameter before demarcating and planting . After land preparation, low-lying
beds, 30 cm high, 5.6 m wide and 2.6 m long, were raised. Stem cuttings with
a length of 30 cm (5 to 8 nodes) were used as planting material and were
placed in rows after each other with a space of 35 cm between rows. The
cuttings were covered with soil between the nodes, which were at a distance
of 6 cm for Hshinchu variety and 4 cm for Trang giay and Duyen ngoc
varieties.

The plants were watered two times per day in the dry season, and weeding was
done manually every week and pest prevention was done when the pest was
evident.

Data collection and chemical analysis

The first young shoots of the sweet potato vines were cut at 20 days, and
the first harvest was performed at 45 days. The following harvests were done
at 65, 80, 95 and 110 days after planting, respectively, and the last
harvest was at 125 days after planting. One row around the subplots was left
when harvesting and all harvesting was done during dry days. Incidences of
diseases and pests were monitored during the period of the experiment.

The soil samples were analysed at the laboratory of the Soil Science
Department, College of Agriculture, Cantho University. Total N, P, pH,
organic matter (OM) and bulk density were analysed according to the Walkley
Black procedure (Walkley et al, cited by Begheijn 1980). Exchangeable K was
analysed according to Houba et al. (1995).

Fresh weight of the sweet potato vines of each harvest was recorded
immediately after cutting and 2 kg per subplot was randomly collected for
chemical analysis. The samples of sweet potato vines were analysed for DM,
ash, crude protein (CP), neutral detergent fibre (NDF) and acid detergent
fibre (ADF). The DM, CP and ash were determined according to AOAC (1990). CP
content was analysed by the Kjeldahl method as N * 6.25 and ash was assayed
by incinerating samples at 600°C. The contents of NDF and ADF were
determined according to Van Soest and Robertson (1985).

Statistical analysis

The data were analysed by an analysis of variance using the General Linear
Model (GLM) procedure of Minitab Statistical Software Release version 12.21
(Minitab 1998). When the differences between treatment means were
significant at the probability level of P<0.05, the means were compared
using Tukey's pair wise comparison test. The yields of DM and CP in kg per
ha are the sum of all harvests during the period of the experiment. The
statistical model used was Yijk = µ+ bi + Fi +VJ< + FVik + eii where Yijk =
observed yield for treatment j in block i, µ = overall mean, bi = mean yield
for block i, Fi =fertiliser effect on yield for treatment j, Vk = variety
effect on yield for treatment k; FV.ik = interaction between fertiliser and
variety, and eii =represents the random unit variation within a block.

Results and Discussion

The total N, P and exchangeable K contents of the soil
were 0.20, and 0.27% and 0.32Meq/100mg , respectively (Table 1). The OM
content in the soil was relatively high.

Table 1.
Chemical and physical properties of the soil (LS-means and SE, n=6)

Site no.

N(%)

P(%)

K*(Meq/100mg)

OM(%)

pH

Bulk density
(g/cm3)

1

0.17

0.28

0.15

3.54

5.64

1.22

2

0.22

0.32

0.28

4.52

5.53

1.24

3

0.21

0.28

0.49

4.25

5.46

1.26

4

0.21

0.27

0.25

4.63

5.48

1.08

5

0.19

0.29

0.43

3.90

5.41

1.23

6

0.22

0.29

0.32

4.16

5.54

1.20

Means

0.20±0.01

0.27±0.01

0.32±0.12

4.17±0.40

5.51±0.34

1.21±0.06

Exchange K, Meq/lOOmg; total N, J>, OM, % of sample

The nutrient content of goat manure showed a large variation between samples
taken during the experiment. The content of total N was low (2.20% to 3.83%
in DM) while P and K content were quite high, 1.4 and 1.3%, respectively.

The data in Table 1 show the homogeneity of soil in the experiment. This
soil is characteristic for most of the arable land of the Mekong Delta
region. Tiem et al (2000) reported that most arable soils along the Tien and
Hau rivers contained more than 0.18% total N and were rich in P (>0.1%) and
OM (4 to 5%). These findings are also supported by Ung (1974) and Tiem et al
(2000) as levels suitable for the growth of sweet potato, though the pH
value was slightly low at 5.51. Danh (1998) grew soybean at the same site
and recorded the same pH value and confirmed that the pH value did not
affect the vegetative development and yields of many annual crops like
cassava and sweet potato.

Goat manure
had a positive effect on the soil characteristics as bulk density was
decreased, which indicated increased water holding capacity and content
of OM (Duarsa 1996; Martin 2003). The total N, and P and exchangeable K of
the treatments fertilised by goat manure were 0.29, and 0.32%, and 0.53
Meq/100mg, respectively, and were significantly different to the treatments
with chemical fertiliser (0.19, and 0.24% and 0.18 Meq/l00mg, respectively).
The N, P, K, and pH values of soil fertilised by goat manure were also
higher and significantly different to the soil fertilised by chemical
fertiliser or without fertiliser. Duarsa (1996) showed that goat manure can
substitute part of N P K fertiliser and had a good effect on soil
properties. Peacock (l 996) stated that soil benefits from the release of
nutrients in goat manure, and the build-up of organic matter improves the
structure of the soil.

There was no interaction between varieties and fertilisers, and there was no difference
between the three varieties of sweet potato (Table 2). The DM, CP and NDF yields of
the sweet potato vines fertilised by goat manure were not different to the sweet
potato vines fertilised by chemical fertiliser. The yields of sweet potato vines in the control plots were
lower than in the fertilised plots. There was no relationship
between growth period and yield (R2 = 0.014) during the time
interval studied. Yields of sweet potato vines
started to decline at the sixth cutting at 125 days (Figure 1).

Figure 1.
The relationships between the growing time of plants and yield.

The highest DM content was obtained in the treatment without fertiliser, significantly different to the treatment with goat manure
. The sweet potato vines fertilised by chemical fertiliser had the
highest CP content (18.00%), significantly different to the other
treatments. Sweet potato vines fertilised with goat manure had significantly
higher ash content (10.57%) compared to the other two treatments. There were
no significant differences in NDF content between the treatments, whereas
the ADF content of sweet potato fertilised by goat manure was highest and
significantly different to the other treatments (P<0.001). There were three
main factors affecting the yield of sweet potato vines in the experiment:
variety, soil, and fertiliser. The soil was as discussed earlier homogenous,
and there was no interaction between variety and fertiliser and variety had
no effect on yields and chemical composition of sweet potato vines, so the
differences between the treatments were an effect of type of fertiliser.
Apparently, the DM, CP, NDF, ADF yields of sweet potato vines fertilised by
chemical fertiliser and goat manure were highest, but they were not
significantly different to each other. This can be due to appropriate
amounts of soil moisture being present and that the goat manure could have
decomposed under the activities of bacteria to release adequate nutrients
for plant growth even a short time after application (Martin 2003).

Ruiz (1981) showed that some varieties of sweet potato can be grown two or
three times per year and can produce up to 125 tonnes of fresh biomass, of
which 64% are vines, which contain 12% to 17% of CP. During the 125 days of
the present experiment, there was a small variation in yields, but after the
sixth harvest the yields started to decline. Purseglove (1982) confirmed
that sweet potato is a short­ day plant and photosynthesis of the leaves
affected the overall productivity, even the flowering. The observations on
the germination of young shoots after the fifth harvest showed that only a
few young shoots were visible on the sweet potato stems along with the
second and/or the third shoot, which looked old. A level of fertilisation
from 0 to 180 kg of N increased the forage production from 6.7 tonnes DM/ha
to 9.1 tonnes DM/ha (six cuts at 45-day intervals), and from 5.4 tonnes
DM/ha to 6.2 tonnes DM/ha at a cutting interval of 135 days (Quispe 1997 in
Leon-Valarde 2000). There were only a few tubers at the last harvest for all
three varieties. This means that with regularly cutting of the vines at
short intervals the sweet potato cannot produce tubers as well. Ruiz et al
(1980) and Leon-Valarde (2000) confirmed that root production of sweet
potato was markedly reduced when defoliated regularly.

The DM, CP, NDF, ADF content in sweet potato vines did not show any
variation due to treatment (Table 3).

An (2003) reported similar results for DM, CP,
NDF and ADF, but when separately analysing CP of leaves found a higher value
by the effect of fertiliser. Orodho (2003) studied two different varieties
of sweet potato and showed that sweet potato vines contained 17.5% CP, 28.0%
NDF, and 22.0% ADF in DM, and that these values were not different between
the two varieties. Dominguez (2000) reported similar results on nutrient
composition of sweet potato vines of 14.2% DM, 18.5% CP, 12.5% ash, 26.2%
NDF and 22.3% ADF (percent of DM) and concluded that the vines had a lower
carbohydrate content, but were higher in fibre and protein in comparison to
roots, and could be used as a source of vitamins and protein. In this study
the ash content of sweet potato fertilised with goat manure was
higher compared with the other treatments. This can be due to
the fact that the goat manure was rich in minerals in a form that the plants
can easily take up.

In general, the phenotypes of plants will partly influence the yield and
nutrient value. In this case the three varieties of sweet potato are
different in phenotype and production type. Trang giay and Duyen ngoc are
typical for tuber production, and are short cycle cultivars with lobe leaves
(small and medium area leaves) and high tuber yield, whereas Hshinchu is
typical for forage production (entire and big leaf), can be grown all year
round in the farmers' backyard, and high yields of vines have been recorded.
The results of the present experiment, however, showed no influence of
variety on yields and chemical composition. The recordings of pest
incidences showed very little pest damage on the sweet potato vines. The few
insects occurring during the experiment could have been from earlier
harvests of vines. Leon-Valarde (2000) confirmed that sweet potato has few
crop pests and diseases, and provides good ground cover for soil
conservation.

Conclusions

Sweet potato produced a considerable amount of vines within a four month
cropping cycle with sequenced harvests of 15 to 20 days intervals. The
nutrient value of the vines was stable both in regard to time and variety.

Goat manure has an effect on soil characteristics, especially organic matter
and bulk density, that is advantageous for the growth of the sweet potato
plant.